The present invention relates to over-current protection circuits and more particularly to counting over-current events over one or more PWM periods to distinguish between noise and real over-current events.
Many power circuit applications require sensing of over-current conditions to protect components of the circuit. Such circuits typically comprise circuit 10, illustrated in FIG. 1, having a gate driver circuit or control IC 12 for controlling a bridge stage 14. The half bridge stage 14 is formed of high and low switches Q1 and Q2 connected at a node N. An inductor L1 is connected between the node N and a node M, which connects series connected capacitors C1 and C2. The switch Q2 is connected between the node N and a resistor Rsense.
Typically, when a signal A, as measured at the resistor Rsense, is provided to the control IC 12 for determining an over-current condition. A comparator circuit 18 may be used to compare the signal A to a user provided preset threshold VTH. When the provided threshold VTH is exceeded, the control IC 12 enters a protection mode. This usually results in some modulation of a signal Vg of the control IC 12 sent to the switch Q2 or, more typically, shutting down of the whole circuit 10.
The majority of the power circuits today operate in noisy environments. This noise includes switching noise from other functional circuit blocks, Electrostatic Discharge (ESD), or even thunder strike in automotive applications. Transient noise spikes can be on the order of 10 s or 100 s of ns and resemble over-current signals if processed.
Conventionally this is most easily accomplished by using a blanking filter circuit 16 after the over-current comparator. In this approach, the over-current signal B must last longer than a filtering time period allowed the blanking filter circuit 16 for registering the over-current signal B. Noise spikes shorter than a length of the filtering time period are automatically rejected. To prevent such misfire in the worst scenario, it is not sufficient for the blanking filter circuit's filtering time period to be longer than most noise spikes—the filtering time period must be longer than ALL noise spikes. This restriction results in a lengthy filtering time period, which has the following drawbacks:    1. The blanking filter circuit filtering time period can exceed the ON time of the switch Q2, especially when the duty ratio is low. This is a serious restriction in variable duty systems such as Class D amplifier where over-current protection is only active when the ON time is longer than the filtering time period.    2. If over-current condition occurs towards the end of the ON period, it will be filtered out and not registered until the next cycle.    3. A current in the inductor L1 is unchecked and switches un-protected for the length of the blanking filter circuit's filtering time period. The inductor current can ramp up to an unacceptable levels (saturation) when the blanking filter circuit's filtering time period becomes long.
In a very noisy environment where multiple channels are switching asynchronously, the length of the filtering time period required for the blanking filter circuit 16 can be so long that it exceeds the switching period, rendering the over-current protection method impractical.
Alternatively, in another approach, when over-current condition is detected, the switch current constant can be held to only invoke the shut down mode after a fixed time period (typically in the ms. range). This would avoid immediate reaction to noise signals. However, there is a chance that the system will stay in a constant current mode and never enter the shut down mode. This scenario can happen in audio applications where the length of the fixed time period is longer than a half cycle of an audio signal. This undesirable loophole can lead to overheating of the switch and an eventual system failure.